Gap junctions which comprise a family of proteins called connexins have been implicated in the morphogenesis of the chick limb bud. We have examined the expression patterns of two members of the connexin family, connexin43 (Cx43) and connexin42 (Cx42), during the early development of the chick limb bud and embryo by in situ hybridization. Cx43 mRNA is expressed in high amounts in the apical ectodermal ridge (AER), which promotes the outgrowth of the mesodermal cells of the limb bud, and in the ectopic AER of the limb buds of polydactylous diplopodia-5 mutant embryos. In contrast, little Cx43 expression is detectable in nonridge limb ectoderm at early stages of limb development. These results suggest that Cx43 gap junctions may integrate the activity of the cells comprising the AER and compartmentalize them into a functionally distinct entity capable of directing limb outgrowth. In addition, Cx43 exhibits high expression in the posterior subridge mesoderm of the early limb bud that is growing out in response to the AER, but little expression in the anterior mesoderm. This graded distribution of Cx43 transcripts correlates with a functional gradient of gap junctional communication along the anteroposterior (AP) axis, and suggests that Cx43 gap junctions may be involved in pattern formation across the AP axis. At later stages of development, Cx43 is transiently expressed in high amounts in the precartilage condensations of the carpals and metacarpals, at a time when critical cell-cell interactions are occurring that trigger cartilage differentiation. In contrast, in the developing limb, Cx42 is expressed exclusively by the central artery. In the remainder of the chick embryo, Cx42 is expressed in high amounts by the vessels comprising the arterial vasculature, but is not expressed by the venous vasculature. Thus, Cx42 gap junctions may be involved in specification of the arterial vasculature of the limb and embryo. Cx42, but not Cx43, is expressed in the ventricle of the heart, and by cells along the intrasclerotomal fissure that separates the rostral and caudal halves of the sclerotome of somites into distinct communication compartments.